This invention relates to a track access control method and apparatus for an optical disk apparatus, and more particularly to a track access control method and apparatus for an optical disk apparatus wherein two beams including a write beam and a read beam pass through a single object lens of an optical head such that, while data are being written onto a certain track of an optical disk with the write beam, the information written with the write beam is read out with the read beam which is positioned rearwardly of the write beam in the direction of rotation of the disk.
Verify read is carried out with conventional optical disk apparatus. Such verify read involves reading, after data have been written onto an optical disk, of such data to compare the read out data with the write data to assure the reliability of the optical disk apparatus.
In conventional systems, a single beam of light is projected onto an optical disk by way of an object lens of an optical head so that it may serve for both of writing and reading. More particularly, using a single beam of light, writing is carried out for a certain one rotation of an optical disk, and then, reading is carried out for a next one rotation of the optical disk, whereafter the write data and the read data are compared with each other. With an optical disk apparatus which carries out such verify read, an optical disk must be rotated by two complete rotations for the writing of data, and there is a problem that much time is required.
In recent years, a technique has been proposed wherein a write beam and a read beam pass through a single object lens of an object head at the same time such that, while data are being written onto a certain track of an optical disk with the write beam, the information written with the write beam is read out by the read beam which is positioned rearwardly of the write beam in the direction of rotation of the disk. Consequently, writing of data and verify read are carried out at the same time thereby to reduce the data writing time of the optical disk apparatus. In the optical disk apparatus, focusing of a beam of light is controlled by a focusing servo controller while the track position of the beam is controlled by a track servo controller simultaneously for both of the write beam and the read beam. Track servo control makes use, for example, of a variation of the amount of reflected light due to a diffraction phenomenon of a beam spot by a spiral guide groove (track) provided in advance on an optical disk. In particular, making use of the fact that a reflected light amount distribution detected by a photodetector is varied with a diffraction of light by a track depending upon a position of a beam spot relative to the track, a track error signal of a beam spot with respect to the track is obtained.
Such track error signal is obtained by receiving an amount of reflected light of a write beam by means of a photodetector. Conventionally, in an optical disk apparatus wherein two beams of light are projected onto an optical disk through a single object lens to carry out writing and reading of data at the same time, a track error signal is obtained only from the write beam to control the track positions of the two beams at the same time.
In such optical disk apparatus, when a certain track of an optical disk is to be accessed, an RF signal is produced by a photodetector which receives a write beam of light, and an address of one of tracks preformatted upon production of the optical disk at which a write beam is positioned at present is read from the RF signal. Subsequently, a difference is detected between the thus read track address and a target track address designated from a supervisory controller so as to be accessed, and while reading an RF signal from a write beam, an optical head is moved by an optical head drive motor. Then, when the target track address is reached, the optical head drive motor is controlled to stop the optical head.
In an optical disk apparatus of the type wherein an object lens mounted on an optical head moves, upon accessing thereof, on a straight line which passes the center of an optical disk, when the optical head moves so that a write beam therefrom accesses a certain track, a read beam moves to simultaneously access the same track.
In recent years, a novel optical disk apparatus has been proposed wherein an optical head has, in addition to an object lens for projecting a write beam and a read beam onto an optical disk, another object lens mounted therein for projecting an erase beam onto the optical disk. In such optical disk apparatus, it is impossible to move both object lenses, upon accessing by the optical head, on a straight line which passes the center of the optical disk. Accordingly, such optical disk apparatus must necessarily have such a construction that the object lenses move on a straight line which is offset from the center of the optical disk. It has been apparent that, in an optical disk apparatus of the type just mentioned, since the relationship between a track position of a write beam and another track position of a read beam is different on the inner side and the outer side of an optical disk, it is necessary to correct the track position of the read beam in accordance with the access position of the optical disk.